A NEARBY OLD HALO WHITE DWARF CANDIDATE FROM THE SLOAN DIGITAL SKY SURVEY

Abstract

Abstract : We report the discovery of a nearby old halo white dwarf (WD) candidate from the Sloan Digital Sky Survey (SDSS). SDSS J110217.48+411315.4 has a proper motion of 1'.75 yr-1 and redder optical colors than all other known featureless (type DC) WDs. We present SDSS imaging and spectroscopy of this object, along with near-infrared photometry obtained at the United Kingdom Infrared Telescope (UKIRT). Fitting its photometry with up-to-date model atmospheres, we find that its overall spectral energy distribution is fit reasonably well with a pure-hydrogen composition and Teff H 3800 K (assuming log g = 8). This temperature and gravity would place this WD at 35 pc from the Sun with a tangential velocity of 290 km s 1 and space velocities consistent with halo membership; furthermore, its combined main-sequence and WD cooling age would be H11 Gyr. However, if this object is a massive WD, it could be a younger object with a thick disk origin. Whatever its origin, the optical colors of this object are redder than predicted by any current pure-hydrogen, pure-helium, or mixed hydrogen helium atmospheric model, indicating that there remain problems in our understanding of the complicated physics of the dense atmospheres of cool WDs.